>It is my understanding that the chemicals in marijuana are even more conducive to causing lung cancer than tobacco.

I did a research paper on it back in high school. One study found that, ounce for ounce, pot had 3 times as many carcinogens, and 5 times as much tar as tobacco. I forget exactly what the relative frequency of smoking vs pot use was for the average user, but it was something like 2 joints vs almost a pack a day of cigs.